It's a traditional Hawaiian song. Two versions of it are used on the show. The first version is performed by Hans Haider and the second version by George De Fretes. This theme plays a lot thru season 1. Also played in a few episodes from seasons 7 & 8. Title card for Party Pooper Pants and I Had an Accident. It also plays in a newer episode What's eating Patrick?
George De Fretes Version: http://youtu.be/Y8JgpgsaJBY